1. Field of the Invention
The present invention relates generally to oxidization of organic materials in a multiple hearth furnace and, more particularly, to the incineration of sludges and the reactivation of spent activated carbon.
2. Description of the Prior Art
Conventionally, sludge is burned in a multiple hearth furnace ("MHF") in the countercurrent mode; this is illustrated in FIG. 2. Typically, there is a sludge drying zone, a sludge burning zone and an ash cooling zone; air and combustion products travel countercurrently to the sludge and ash. Often, an afterburner is required to oxidize organic materials in the exhaust gases.
Difficulties with this system have become apparent as sludge dewatering has improved over the last few years, resulting in a drier sludge fed to the MHF. This dry sludge can burn at too high a temperature causing damage to the furnace, volatilization of heavy metals, and melting of ash to form slag. The burning process is also unstable because unavoidable small variations in the sludge quality or feed rate cause large variations in furnace operation.
Prior attempts at overcoming these difficulties have included: pyrolysis (starved air operation) and excess air operation. Pyrolysis or starved air operation involves reducing the amount of air fed to the furnace to control the burning rate. This process has proved difficult to control. With excess air operation, a large excess of air is passed through the furnace to reduce the temperature. Excess air operation is inefficient in terms of fuel consumption and places an extra load on the off-gas cleaning system. The burning process is unstable requiring constant operator attention. This is the way many furnaces are currently operating.